U.S. regulators exempted BP Plc from a detailed environmental review of the exploration project that ultimately resulted in the deadly Gulf of Mexico explosion and subsequent oil spill, documents show.

The Minerals Management Service granted BP's project a "categorical exclusion" from full environmental analysis normally required under the National Environmental Policy Act, according to documents made available by the Center for Biological Diversity, an environmental group.

BP had argued in a letter to the Council on Environmental Quality (CEQ), a White House agency, last month that the use of the exclusion for projects did not mean environmental impacts were being ignored, only that an agency agrees they are deemed to be minimal or nonexistent.

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The MMS, the branch of the U.S. Interior Department that is responsible for managing oil, gas and other resources on the outer continental shelf, approved the exploration project on April 6, 2009.

The exclusion puts pressure on President Barack Obama's administration to show it could not have done more to prevent what may become the most damaging oil spill in U.S. history.

Kieran Suckling, the environmental group's executive director, said the incident showed the Obama administration's support for increased offshore drilling had obscured Interior Secretary Ken Salazar's pledge to reform the MMS.

"Instead of protecting the public interest by conducting environmental reviews, his agency rubber-stamped BP's drilling plan, just as it does hundreds of others every year in the Gulf of Mexico," Suckling said in a statement.